Since C++14, there is a spec'ed attribute for deprecation of methods,
functions, types, aliases or anything else.
This CL switches from the GCC __attribute__ to this standard attribute.
This allows to use the V8_DEPRECATED and V8_DEPRECATE_SOON macros on
anything where the standard attribute can be used (including {using}
statements that were not working before). It also avoids the need to
nest the whole declaration in the macro, making the code more readable.

R=adamk@chromium.org

Bug: v8:9810
Change-Id: I7adab7694af75423fb31ade2fc982dbf9c9bc699
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1847361Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64174}